1. Field of the Invention
The present invention relates to a multi-section Roots vacuum pump of the reverse flow cooling type with internal coolant water passages. The present invention is applicable to a reverse flow cooling type multi-section Roots vacuum pump which is operated at a high compression ratio in the range from atmospheric pressure to 10.sup.-3 Torr at a relatively high temperature.
2. Description of the Related Arts
In general, in a Roots type vacuum pump in which rotor pairs rotating in a housing to draw in and discharge gas have a minute clearance from the housing which accommodates the rotor pairs therein, it is important that the clearance between the rotor and the housing be as small as possible in order to realize a pump having a high performance.
In a prior art multi-section Roots vacuum pump driven at a high compression ratio, the temperature will rise relatively high due to the compression heat during operation, and a jacket is arranged directly around the housing which accommodates the rotor pairs therein, to protect the pump from superheating by coolant water running through the jacket for cooling the pump by the radiation of compression heat to the open air. However, since the housing is directly cooled by coolant water, the temperature of the housing in the operating state of the pump becomes significantly low in contrast to the temperature of the rotor pairs inside the housing, thus the clearance between the housing and the rotor pairs is reduced because the amount of thermal expansion of the housing becomes smaller than the amount of thermal expansion of the rotor pairs, and there is a possibility of a contact between the housing and the rotor. To prevent such contact from occurring, the clearance between the housing and the rotor pairs should be preset larger than preferred. This situation is an obstacle to the realization of a pump having a high performance by minimizing the amount of gas leakage from the clearance mentioned above.
Further, as disclosed, in another prior art reverse flow cooling type multi-section Roots vacuum pump, the pump includes a connection pipe provided to connect the outlet passage of a specific pump section with the inlet passage of the following pump section, a cooler incorporated to the connection pipe, and a reverse flow pipe branched off from the connection pipe at the downstream side of the cooler and arranged to lead the reverse flow cooling gas to the preceding pump section. Reference can be made to Japanese Unexamined Patent Publication (Kokai) No. 59-115489, and Japanese Unexamined Patent Publication (Kokai) No. 63-154884.
In the reverse flow cooling type multi-section Roots vacuum pump, a plurality of external coolers is provided for cooling gas running through the connection pipe to protect the pump from superheating by radiating compression heat produced at each pump section. Further, an external piping arranged outside the pump consists of connection pipes for connecting the outlet of each pump section and the inlet of the following pump section, and reverse flow pipes branched off from the connection pipes for leading reverse flow of coolant gas to the preceding side pump section. Therefore, this relatively complicated structure of the external piping arrangement is not advantageous from the viewpoints of compactness of the pump and manufacturing cost of both the external cooler and the external piping. Accordingly, a realization of a small sized pump having a high operation performance has been strongly desired.